A gate valve has a body with a central chamber that is intersected by a flow passage. A gate moves within the chamber between the open and closed positions. The gate has a hole through it that aligns with the flow passage while in the open position. The gate may be of a split type, comprising two halves or it may comprise a single slab. A stem extends into engagement with the gate for moving the gate between open and closed positions. The chamber has a central portion, which is intersected by the flow passages, and a stem portion that extends from the central portion. Typically, a seal in the stem portion of the chamber engages the stem to seal pressure within the chamber.
In one type, the stem extends into rotatable engagement with a threaded nut or sleeve secured to the gate. Rotating the stem causes the gate to move linearly. In another type, the stem does not rotate. A threaded nut sleeve mounted in the bonnet of the valve engages the stem, and when rotated, causes the stem to move linearly. The threads of the sleeves and stem may slide against each other, or they may employ balls between the grooves for reducing friction.
Gate valves may be operated manually, such as with a wheel mounted to the stem or the nut sleeve. It is also known to utilize a remote operated vehicle (ROV) to engage and rotate a stem or nut sleeve. Hydraulically powered actuators are also utilized wherein a piston moves the stem linearly without rotation. Electrical actuators are also known that employ an electrical motor and a gear train to rotate a stem or nut sleeve to cause movement of the gate.
With actuators, typically, a spring is used to return the gate to a fail-safe position if power is lost. The fail-safe position could be either an open or a closed position. In a subsea environment using a hydraulic actuator, the spring must be strong enough to overcome hydrostatic pressure when pushing the piston back, requiring accommodation of relatively high loads and adversely impacting the physical size (and weight) of the actuator accordingly.